1. Field of the Invention
The present invention relates generally to an apparatus for polarizing a piezoelectric body such as piezoelectric ceramics, and more particularly, to a piezoelectric body polarizing apparatus capable of polarizing a piezoelectric body while measuring the degree of polarization thereof.
2. Description of the Prior Art
In manufacturing a piezoelectric component such as a piezoelectric resonator or a piezoelectric filter, a piezoelectric body such as piezoelectric ceramics must be polarized. Piezoelectric ceramics have been conventionally polarized by the following processes.
First, electrodes for polarization are formed on both surfaces of a piezoelectric body, and a DC voltage of, for example, several kilovolts is applied to the piezoelectric body to polarize the piezoelectric body. A high frequency voltage is then applied to the piezoelectric body, to measure impedance-frequency characteristics, for example. The degree of polarization is evaluated from the frequency difference between a resonance point and an antiresonance point, for example, to confirm whether or not a desired degree of polarization is attained. If the measured degree of polarization is not the desired degree of polarization, a DC voltage is applied again to adjust the degree of polarization. For example, if the attained degree of polarization is lower than the desired degree of polarization, the DC voltage is applied to the piezoelectric body in the same direction as the DC voltage applied in the first process of polarization to increase the degree of polarization. On the other hand, if the attained degree of polarization is higher than the desired degree of polarization, the DC voltage is applied to the piezoelectric body in the opposite direction to the DC voltage applied in the first process of polarization to decrease the degree of polarization. Thereafter, the impedance-frequency characteristics, for example, of the piezoelectric body are measured again, thereby to confirm whether or not the degree of polarization attains the desired degree of polarization. The above described processes of measuring the degree of polarization and controlling the degree of polarization are repeated, thereby to obtain a piezoelectric body having the desired degree of polarization.
In the piezoelectric body, the attained degree of polarization is found to significantly vary even if the same DC voltage is applied, due to the variation between the manufacturing lots. In the conventional method of polarization, therefore, the processes of polarization and controlling polarization and the process of measuring the degree of polarization must be repeated a significant number of times, resulting in very complicated work.
If the process of polarization and the process of measuring the degree of polarization were to be simultaneously carried out, it is considered that a piezoelectric body having the desired degree of polarization could be easily obtained without passing through the above described complicated processes. However, a very high DC voltage of several kilovolts is applied in the case of polarization. If a high frequency circuit for measuring the degree of polarization were connected to the piezoelectric body, therefore, it is considered that the effect of the high DC voltage on the high frequency circuit could not be avoided. Consequently, no attempt to apply a DC voltable for polarization to the piezoelectric body and to measure the degree of polarization of the piezoelectric body at the same time has been conventionally made.